With the demand increase of the mobile communication users, high spectrum efficiency becomes one of the main requirements of the mobile communication system, and orthogonal frequency division multiplexing (OFDM) is just one main support physical layer technology. In the corresponding Orthogonal Frequency Division Multiplexing Access (OFDMA) under the multi-user environment, more than one mobile terminals can multiplex spectrum resources at the divided subcarrier resource level, for example, the downlink of the long-term evolution (LTE) system uses such OFDMA technology.
FIG. 1 shows a structural block diagram of a broadband wireless OFDMA base station transmitter. The data of each user are encoded and modulated, and the base station maps the signals in the broadband frequency range by multi-user scheduling. By using OFDMA technology, the broadband spectrum can form N parallel subcarriers to carry data, for example N is 2048, and the base station can perform optimized scheduling for each user over these N subcarriers.
As to one certain user, the base station will generally obtain the information about the wireless channel from the base station to one certain user (i.e. mobile terminal) via the channel quality feedback of the mobile terminal or using the reciprocity of the uplink/downlink channel of the time division duplex (TDD), and the base station can allocate the subcarriers which are suitable for the receiving by a user to this user (the fading is relatively small) so as to achieve the object of improving user throughput. The base station can allocate consecutive subcarriers to the user and can also allocate non-consecutive subcarriers to the user.
FIG. 2 shows an example of subcarrier allocation within one frequency band. In a base station transmitter, a part of frequency domain subcarriers are used as reference training signals, and what is sent is confirmed known signals, and another part of frequency domain subcarriers are used for carrying service data to be transmitted. The reference signal subcarriers can assist the receiver to perform channel estimation so as to complete the coherent demodulation of data.
In addition to this, some subcarriers carry relevant control channel information, and the base station uses relevant control channels to inform each user of the scheduling information and the parameters (such as encoding rate, modulation mode, etc.) of each service channel timely. In addition, some common control channels, such as synchronization signal, broadcast channel, etc. will also occupy a certain amount of frequency domain subcarrier resources and they are mapped in the system frequency band together with the subcarriers of other channels without overlap.
After mapping, each symbol in the whole frequency band will do Inverse Fast Fourier Transform (IFFT), and the frequency domain signals are transformed into time domain. After being added with a cyclic prefix as the protection of anti-OFDM inter-symbol interference, the signals are transmitted to the whole cell from the antennae via modules such as Digital Analog Converter (DAC) module, radio frequency (RF) module, etc.
In the OFDMA system, each mobile terminal receives downlink signals, and the block diagram of the receiver of the mobile terminal is as shown in FIG. 3. The air port signals are transformed onto the baseband by the radio frequency module RF and sampled as baseband signals by the Analog Digital Converter ADC module, and the cyclic prefix is removed over time domain, and then the time domain OFDM symbols are sampled to complete Fourier FFT transform to obtain the data of frequency domain; and in the de-mapping module, each mobile terminal takes out the subcarriers which belong to itself and relevant reference subcarrier signals. There is special control channel or control signaling in the broadband system for informing each user of the location of the time frequency resource allocated thereto by the base station. In the following, the signal estimation module uses the reference subcarrier signals to estimate the frequency domain subcarrier channel from the base station to the user; and the demodulation module carries out coherent demodulation on the data subcarrier signals according to the channel estimation value to obtain corresponding data bit judgment value. The same is sent to the decoding module for decoding and finally the data information is obtained.
In the conventional receiver used for broadband system, the modules such as radio frequency module RF, analog digital converter ADC module, Fourier transform module FFT, etc. all work under high data rate mode, and the user will carry out receiving with rather large power consumption. However, most of the time, the user does not have high speed data service, for example, the effective data bandwidth of this user only occupies 180 kHz, and the conventional receiver used for broadband system will still receive this signal by manner of performing the receiving over all the bandwidth, and the receiver still remains in the receiving state with rather high processing load, which will consume quite a lot of battery energy undoubtedly. As to the portable equipment of the user, how long the working time of the battery is will closely impact the user experience of the mobile communication products.